Turbo charging an engine allows the engine to provide power similar to that of a larger displacement engine while engine pumping work is maintained near the pumping work of a normally aspirated engine of similar displacement. Thus, turbo charging can extend the operating region of an engine. Turbochargers function by compressing intake air via a turbine operated by exhaust gas flow. During vehicle launch conditions, such as when accelerating from idle, minimal exhaust gas flow combined with increased load on the compressor results in a delay of the throttle response, referred to as turbo lag, leading to reduced engine power output.
One example approach to reducing turbo lag is described by Dixon et. al. in U.S. 2003/01106541. Therein, the likelihood that engine torque will need to be boosted is estimated based on engine operating parameters, and an idle speed of the compressor is controlled as a function of the torque boost likelihood.
However, the inventors herein have identified potential issues with the above approach. Controlling the idle speed of the compressor requires a separate motor to power the compressor. Operation of the motor reduces engine efficiency, thus wasting fuel, and requires extra engine packaging space.
Thus, in one example, some of the above issues may be at least partly addressed by a vehicle launch method. The method comprises, at idle before pedal tip-in, increasing alternator load and boost pressure while maintaining idle speed, and responsive to pedal tip-in, decreasing alternator load.
In this way, before vehicle launch initiated by a pedal tip-in, the vehicle can be “pre-boosted,” or operated to increase boost to increase engine power output during the launch. In one example, the pre-boosting includes placing a maximum load on the alternator and retarding spark timing. In some embodiments, pre-boosting may also include adjusting intake valve timing to best volumetric efficiency. By doing so, the engine air flow rate may be increased and the extra energy may be routed to the exhaust via the retarded spark timing and/or the extra energy may be sent to electrical storage via the alternator. Exhaust output will thus increase, resulting in increased turbine spinning and increased boost. Further, the load on the alternator may be reduced or completely unloaded and spark timing may be advanced during the vehicle launch in order to increase acceleration torque.
The present disclosure provides several advantages. By both pre-boosting the engine and decreasing alternator load at vehicle launch, turbo lag can be reduced. By reducing turbo lag without the inclusion of an additional motor to power the turbine or compressor, engine efficiency can be improved. Additionally, in hybrid vehicles, the extra energy stored as a result of increasing the load on the alternator may be discharged during or following the vehicle launch and used to power the motor, further increasing engine efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.